[cff] Fix memory overflow.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=9869
  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10869

* src/cff/cffparse.c (destruct_t2s_item, cff_parser_run): Store
evaluated T2 charstrings in separately allocated memory.
2 files changed